Typically, semiconductor laser diodes are stable at a particular wavelength over a very narrow range of temperatures, i.e., approximately a few degrees. The reason for this instability is that a number of adjacent longitudinal modes all have approximately the same threshold gain (lasers operate at the wavelength with the smallest net threshold gain). In order to circumvent this problem, laser diodes have been developed which favor longitudinal modes of specific wavelengths. These types of laser diodes are generically called, frequency stabilized laser (FSL) diodes. The two main types of FSL diodes are distributed feedback (DFB) laser diodes (K. Aiki et al., "GaAs-AlGaAs distributed-feedback diode lasers with separate optical and carrier confinement, " Appl. Phys. Lett., vol. 27, pp. 145-146, 1975) and distributed Bragg reflector (DBR) laser diodes (H. Namikazi, M. K. Shams, and S. Wang, "Large-optical-cavity GaAs-(GaAl)As injection laser with low-loss distributed Bragg reflectors" Appl. Phys. Lett., vol. 31, pp. 122-124, 1977). The basic operating principle of these laser diodes is that for a very narrow range of wavelengths (called DFB modes) adjacent to the Bragg wavelength, .lambda..sub.B (see FIG. 1) optical feedback is provided not only by the end-facet mirrors (the Fabry-Perot modes), but also by a periodic variation of the index of refraction along the longitudinal length of the laser. For an ordinary semiconductor laser diode, which only has Fabry-Perot modes, the threshold gain is approximately uniform for nearby wavelengths; whereas, FIG. 1 shows that for a DFB laser diode with a third-order grating, there is approximately a 17% difference between the gain of the mode with the smallest threshold gain and that of its neighbor. Since DFB laser diodes have a larger modulation in the threshold gain compared to that of typical semiconductor laser diodes, these laser diodes exhibit a greater wavelength stability with respect to temperature (on the order of 50.degree. C.). In order to improve the temperature stability, it is necessary to increase the modulation in the threshold gain. The modulation depth can be improved either by increasing the losses of the Fabry-Perot modes with respect to the DFB modes or by selectively decreasing the losses of the DFB modes. The former approach was taken by Anderson et al. (D. Anderson, R. August, and J. Coker, "Distributed-feedback injection laser with fundamental grating," Applied optics, vol. 13, pp. 2742-2744, 1974), who intentionally tilted the end-facet mirrors relative to each other by sawing (resulting in damaged facets), which lead to a substantial increase in the threshold gain modulation depth. However, this approach is not tenable for manufacturing since damaging part of the optical cavity results in a considerable shortening of the lifetime of lasers and higher threshold currents. Another typical method for increasing the Fabry-Perot losses is to place anti-reflection coatings on the end-facet mirrors. However, this method has the disadvantage of resulting in competition between the transverse electric (TE) and transverse magnetic (TM) DFB modes, giving rise to multimode laser diode operation. The latter approach of selectively decreasing the losses of the DFB modes is typically obtained by increasing the depth of the grating, i.e., resulting in a larger index of refraction variation along the longitudinal length of the laser diode. This approach also has its concomitant problems, such as, difficulty in reproducibly obtaining the same grating depth and an increase in the asymmetry of the output beam.